Personally,
I believe that all home inspectors should have a CO detector on their belt. You don't necessarily have to say you checked for CO but if it registers a measurable amount, you can definitely say so.
I use mine to alert the presence, not the absence.

An example of its usefulness:
I had a homeowner tell me before I even started an inspection that I should not expect to find any smoke or CO detectors because they kept going off and she assumed it was paranormal (She also had young children in the house).
When I turned the boiler on, I detected a very large amount of CO in the entire finished basement. The cause turned out to be a tree branch that had fallen into the flue and then was covered with leaves completely blocking the flue (something I surely would have found anyway). The water heater also vented to this. Also, the homeowner smoked so that busted the paranormal myth about the smoke alarms also.

Anyway, it just makes sense to me that you can't be too safe, even if you don't report on it...